The present invention relates to an electronically controllable brake booster. In particular, the present invention relates to an electronically controllable brake booster with a vacuum chamber and a pressure chamber, which are separated from each other by a movable wall, a control valve arrangement which can be actuated by means of an electromagnetic actuation means, and by means of which a pressure difference between the pressure chamber and the vacuum chamber can be adjusted, with the control valve arrangement, as a function of a current flowing through the electromagnetic actuation means, assuming a holding position in which the current ranges between a higher value and a lower value without the control valve arrangement leaving its holding position, a first pressure changing position in which the current is higher than the higher value, and a second pressure changing position in which the current is lower than the low value.
From DE 195 27 493 A1 an electromagnetic actuation means is known which comprises a solenoid coil which can be subjected to a control current and an armature which is associated with the solenoid coil and which is adapted to perform movements which are a function of a control current flowing through the solenoid coil and a spring arrangement which biases the armature in the opposite direction.
A holding position is defined as a manipulated variable, which the armature assumes at a holding current through the solenoid coil. This holding current flowing through the solenoid coil can be changed to a higher value or to a lower value without the armature leaving the holding position.
In addition, both the higher and the lower current value are dimensioned in such a manner that interfering influences on the magnetic and spring forces actuating the armature do not bring the armature into an actuated position which differs from the holding position.
For this purpose, the higher current value is determined in such a manner that a value which is related to the position of the armature in the holding position is determined, the control current is increased by a predetermined current step in a stepwise manner, and the control current is output to the solenoid coil of the electromagnetic actuation means as a manipulated variable, until the value which is related to the position of the armature in the holding position changes by a predetermined value towards the second actuated position.
The lower current value is determined in such a manner that a value which is related to the position of the armature in the holding position is determined, the control current is decreased by a predetermined current step in a stepwise manner, and the control current is output to the solenoid coil of the electromagnetic actuation means as a manipulated variable, until the value which is related to the position of the armature in the holding position changes by a predetermined value towards the first actuated position.
From this document is it also known that the control valve arrangement can reliably be brought into the holding position if the arithmetic mean value of the currents is selected for the holding current.
However, the learning of the working points discussed in DE 195 427 93 A1limited in that the decisive currents for the lower limit and the upper limit must be learned and stored at each commencement of a trip upon switching on the ignition or in periodic time intervals during driving. Upon controlling the control valve arrangement, the currents which have been learned in this manner are used for specifying the first manipulated variable in order to achieve an adequate control behaviour.
However, the pressure difference acting upon the movable wall of the brake booster, which can be adjusted to different values depending upon the desired control, is not considered quantitatively. The pressure difference adjusted at the movable wall is also acting immediately upon the valve body, the valve seat, and the valve element, which also leads to a shift of the currents for the lower limit and for the upper limit.
Thus, the invention deals with the problem which results from the shift of the upper and lower current values that are necessary in order to retain the control valve arrangement in its holding position.
It is therefore the object of the present invention to eliminate this disadvantage so that the control behaviour is further improved in order to compensate interfering influences during operation for a further improvement of the efficiency of the electronically controllable brake booster.
This object is solved with the initially mentioned electronically controllable brake booster in that a value is sensed which is characteristic for the actual pressure characteristic in the pressure chamber, the actual pressure characteristic is checked for the existence of periodic oscillation components of a predetermined frequency band with a predetermined minimum amplitude, and that upon the existence of such periodic oscillation components of the predetermined frequency band, the current for the first pressure changing position is decreased by a first predetermined value, and the current for the second pressure changing position is increased by a second predetermined value.
The inventive method thereby make use of the surprising fact that in an electronically controllable brake booster with incorrectly set working points of the currents for the first and the second pressure changing position, an oscillation frequency occurs which is typical for the respective brake booster, which is superimposed on the signal representing the brake pressure generated in the brake cylinder. It is understood that the predetermined minimum amplitude can be in the near vicinity of zero. However, the interference immunity can be set via this parameter.
This oscillation is detected and, depending on its existence, the values of the currents for the first and the second pressure changing position are changed.
In a preferred embodiment of the invention the decrease of the current for the first pressure changing position is made by the first predetermined value, and the increase of the current for the second pressure changing position is made by a second predetermined value for only a predetermined time period.
After a predetermined time interval following the decrease or increase, respectively, of the current for the first or second, respectively, pressure changing position, the actual pressure characteristic is checked for the existence of the periodic oscillation components of the predetermined frequency band with the predetermined minimum amplitude, and upon the existence of such periodic oscillation components of the predetermined frequency band, the first predetermined value for the decrease of the current for the first pressure changing position and the second predetermined value for the increase of the current for the second pressure changing position are both increased. This is to counteract the vibration tendency of the brake booster.
The check of the actual pressure characteristic for the existence of the periodic oscillation components of the predetermined frequency band with the predetermined minimum amplitude is preferably made by determining the time intervals between characteristic places of the actual pressure characteristic.
Thereby, the characteristic places of the actual pressure characteristic can be one of the maxima, minima, zero crossings, and curve sections with comparable gradients.
In order to be able to find and to evaluate comparable characteristic places of the actual pressure characteristic better, it may be useful to filter out direct components of the signal representing the actual pressure characteristic.
In lieu of the analysis of the actual pressure characteristic in the time range, it is also possible to carry out an analysis in the frequency range (Fourier analysis, etc.).
Primarily, the available computer power or the hardware expenditure to be invested relative to the required or desired accuracy is the decisive factor in which manner the existence of the periodic oscillation components of the predetermined frequency band in the actual pressure characteristic is determined.
In a preferred embodiment of the invention the first pressure changing position is a pressure build-up position (of the control valve arrangement), and the second pressure changing position is a pressure relief position (of the control valve arrangement).
According to the invention, the predetermined frequency band ranges from 0 Hz to 25 Hz, preferably from 5 Hz to 10 Hz, and particularly preferably, ranges about 7 Hz. Primarily, the predetermined frequency band depends on the construction or type, respectively, of the brake booster (moved masses, volumes of chambers, etc.).